Pulsed coherent laser radar systems generally use a single frequency laser to generate a sequence of short transmitter pulses. These are focused onto a distant target, and back-scattered light is collected by a receiving telescope mounted close to, or coaxial with, the transmitting optics. The received light is focused onto a photo-detector where it is mixed with a reference beam from a continuous wave laser, tuned to a frequency close to the transmitter frequency. Mixing of the two signals on the detector surface causes a photo-current to be produced at the difference frequency, which current is filtered and rectified. The rectified signal is a sequence of pulses, delayed with respect to transmitter pulses by the round trip transit time to and from the target. The delay time indicates the target range.
Coherent systems of this kind generally have an advantage in sensitivity over the systems using direct detection of the returned light, in which there is no reference beam and thus no interference pattern on the detector. A disadvantage of pulse coherent systems is the need to have a second laser generating a continuous wave reference beam. The power requirement of this laser is modest, a few milliwatts generally being sufficient, but it needs to be maintained at a frequency close to, but not identical to, the transmitter frequency. This is achieved either by continuously tuning one of the lasers with an electro-mechanical e.g. piezo-electric actuator to maintain a fixed frequency difference, or by locking the two lasers together, and generating a frequency-offset reference beam with an acousto-optic frequency shifter. Either approach requires an optical system and associated control systems of considerable complexity and high manufacturing cost. The optics are sensitive to thermal drift and mechanical vibration, and so are difficult to construct in a sufficiently rugged form for airborne environments.